Partitions used in the diaphragms of turbines, particularly steam turbines, erode over time due to extended use. The erosion occurs principally along trailing edge portions of the partitions and causes loss of performance. It has been the practice in the industry to repair the partitions rather than to effect their replacement or to replace the entire diaphragm. A prior method of repairing steam turbine diaphragms included grinding back the eroded partition, i.e., grinding away trailing edge portions of the partitions, then applying weld material over a copper chill and hand-contouring the weld material by grinding with die grinders. Particularly, the eroded or damaged partition was cut back or ground along the trailing edge until a good parent material became available. The cutback of the original material was typically on the order of 15-25% of the length of the partition from the trailing edge, e.g., as measured along the chord of the partition from its trailing edge to its leading edge. It will be appreciated that the cutbacks or material ground from each partition varied from partition to partition. A thin piece of copper was used as a chill and placed under the area to be welded. Weld material, for example, stainless steel, was then applied to the existing partition. Following the welding, the partitions were ground by hand using burrs and power files to contour each partition by hand back to an approximation of the contours of the original partition. Contouring the pressure side of each partition was a particular problem because of the small entrance opening between adjacent partitions limiting access to the pressure side. Consequently, a small burr grinder was typically used to grind the high spots on the pressure side. However, the pressure side could not be ground precisely to the contour of the original partition but only to a rough approximation thereof, while the suction side could be ground more accurately to the contour of the original partition. Moreover, because the grinding and contouring was performed manually, variations from partition to partition existed even after the repair was complete. It will also be appreciated that this method was time consuming, highly labor intensive because the welds were manually applied and contoured by hand grinding, and oftentimes did not elevate the performance of the diaphragm to its original designed performance.
Subsequently, a method of installing a "similar to" trailing edge section of the partition was adopted. In this method, the partitions were ground back similarly as previously discussed and oftentimes slightly further along the chord, i.e., about 30%. "Coupons" were provided which were similar to the removed trailing edge portions. That is, the coupons were provided in a number of different styles or sizes and frequently did not fit a particular partition perfectly. Problems were encountered with the fit of the coupon to the parent partition because the coupon was not an exact contour of the parent partition and the coupon was often angularly misaligned relative to the parent partition. The coupon required a full penetration weld along its radial length and, following the welding, the partition was hand contoured and stress relieved. Again, however, the hand contouring did not exactly replicate the contour of the original partition.
Later, a further method using an exactly contoured partial partition, i.e., coupon, was installed. Here, the coupon was a precise replication of the trailing edge portion of the original partition up to about 20-30% of the length of the partition, e.g., along its chord. Fit up problems, however, remained similarly as with prior coupons. A full penetration weld was believed necessary to secure the exactly contoured coupon to the remaining portion of the original partition. Because of this, distortion of the trailing edge coupon was encountered and which was a result of the high heat input required by the full penetration weld performed along the radial joint between the coupon and original partition portion. That is, the exactly contoured trailing edge coupon absorbed sufficient heat during application of the full penetration weld along its forward radius such that it would twist and distort during setting necessitating subsequent further grinding and, in large part, defeating the purpose of a trailing edge coupon contoured precisely according to a portion of the original partition.